Soy and Breast Cancer

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Soy and Breast Cancer

Dr. John Johnston
I am commonly asked about soy and its relationship to breast cancer.  Soy is an excellent source of protein and large well designed studies have proven its benefit for the cardiovascular system, bones, and for the relief of Perimenopausal symptoms.  But the burning question that most women have is whether or not they should have soy products, particularly if they have had breast cancer.  Although many studies have shown a decrease in risk of breast cancer with higher soy consumption, there have been studies in laboratories (not humans) that suggest that genistein, a major isoflavone can enhance the proliferation of breast tumor cells.  Another study showed that genistein can promote breast tumor growth in ovariectomized rats.
Soy foods are rich in phytoestrogens, mainly in the form of isoflavones, which are estrogen receptor modulators that have both an estrogenic-like as well as antiestrogenic properties.  Soy constituents have been shown to have anticancer effects, including the inhibition of DNA topoisomerase I and II, proteases, tyrosine kinases, inositol phosphate, and angiogenesis and may boost immune response and possess antioxidant effects.
In December 2009 the Journal of the American Medical Association published an article on soy food intake and breast cancer survival.  It was well designed, and the largest study to date, of over 5,000 female breast cancer survivors.  The conclusion -” Among women with breast cancer, soy food consumption was significantly associated with decreased risk of death and recurrence. “The study went so far as to say that soy food intake may have a comparable effect to tamoxifen on breast cancer outcomes.  The maximum benefit of soy intake for reduction of mortality and recurrence was reached at 11 grams/day.  There was no appreciable benefit beyond this level.
Does the type of soy consumed matter?  Probably. The soy consumed in this study were whole soy foods such as cooked soy beans, edamame, tofu, miso, and soy milk.  In contrast, meat analogs and various processed foods are lower in isoflavones.  Soy supplements should be high quality and organic. I have a great concern about the intake of genetically modified soy.  We may not know the impact of these unnaturally altered soybeans for another generation.  Soy was genetically modified specifically to survive the chemical herbicides that are sprayed over them.  And I have a moral contention with the manner in which certain companies that promote the production of genetically modified soy have overtaken our natural soy farming industry.
Excluding a minority of individuals that may have a soy intolerance, it is my opinion, based on a large and compelling body of evidence provided by legitimate peer reviewed studies, that soy intake is not only safe, but beneficial.

Dr. John Johnston

Yours in health,

Sean M. Fisher,

Functional Medicine Consultant

M.S. – Health, Sport & Exercise Science

FirstLine Therapy® Certified Lifestyle Educator